At the time of reading information recorded on an optical disk or writing information on an optical disk, an optical element to modulate (e.g. polarize, diffract or phase-shift) a laser beam is required.
For example, at the time of reading information, linearly-polarized light emitted from a laser light source is transmitted through a deflecting element and then through a phase plate and arrives at the surface of an optical disk. The polarization direction of the outward linearly-polarized light is aligned in a direction not changed by the deflecting element, and the outward linearly-polarized light is linearly transmitted through the deflecting element and transformed by the phase plate into a circularly-polarized light. This circularly-polarized light is reflected on the recording surface and becomes a reversed circularly-polarized light, which is again transformed by the phase plate into a linearly-polarized light perpendicular to incidence. Such a returning light beam will have its traveling direction bent when it is again passed through the deflecting element, and arrives at a light receiving element.
Further, during reading or writing of information, if the optical disk undergoes plane wobbling or the like, the focus position of the beam spot will be displaced from the recording surface, and a servo mechanism will be required to detect and correct the displacement to let the beam spot follow a concavo-convex pit on the recording surface. Such a servo system for an optical disk is constructed so that the focus of a beam spot irradiated from a laser light source is adjusted on the recording surface and then the track position is detected to let the beam spot follow the desired track. Further, it is also necessary to make sure that the laser beam reflected without hitting the pit on the recording surface will not return as it is to the light source.
For this purpose, in an optical head device, an optical element to modulate (e.g. polarize, diffract or phase-shift) a laser beam is required. For example, a phase plate (wavelength plate) has effects to give a different refractive index to incident light depending upon the angle between the optical axis of the phase plate and the phase plane of the incident light and to shift the phases of two component lights formed by birefringence. The phase-shifted two lights will be joined when emitted from the phase plate. Such shifting of the phase is determined by the thickness of the phase plate. Accordingly, by adjusting the thickness, it is possible to prepare e.g. a quarter-wavelength plate having the phase shifted by π/2 or a half-wavelength plate having the phase shifted by π. For example, a linearly-polarized light passed through the quarter-wavelength plate will be a circularly-polarized light, and a linearly-polarized light passed through the half-wavelength plate will be a linearly-polarized light with its polarized light plane inclined at 90°. Optical elements are combined by utilizing such characteristics and applied to a servo mechanism, etc. Such optical elements are not limited to optical pickup elements used for reading records on optical disks, but they are utilized also for imaging elements in application to projectors, etc. or communication devices in application to wavelength-tunable filters, etc.
Further, these optical elements may also be prepared from liquid crystal materials. Liquid crystal molecules having polymerizable functional groups have both characteristics as a polymerizable monomer and characteristics as a liquid crystal. Accordingly, if the polymerization is carried out after liquid crystal molecules having polymerizable functional groups are aligned, it is possible to obtain an optical anisotropic material having alignment of the liquid crystal molecules fixed. The optical anisotropic material has an optical anisotropy such as a refractive index anisotropy derived from a mesogen skeleton and is applied to e.g. a diffraction element or a phase plate by the use of such a characteristic.
As such an optical anisotropic material, a polymer liquid crystal has, for example, been reported which is obtained by polymerizing a liquid crystal composition containing a compound represented by the following formula (2):
(wherein Q is a 1,4-phenylene group or a trans-1,4-cyclohexylene group, and Z is an alkyl group) (Patent Document 1).
Usually, the following properties are required for optical elements.
1) They have a proper retardation value (Rd value) depending upon usable wavelength and application.
2) The in-plane optical properties (such as the Rd value and the transmittance) are uniform.
3) There is no substantial scattering or absorption at usable wavelength.
4) The optical properties can easily be adjusted to those of other materials constituting the elements.
5) The wavelength dispersion of the refractive index or the refractive index anisotropy is small at usable wavelength.
It is particularly important to have a proper Rd value as mentioned in 1). The Rd value is defined by Rd=Δn (value of refractive index anisotropy)×d (thickness in the light propagation direction). Accordingly, it becomes particularly important that the material to constitute an optical element has a proper Δn value. For example, if Δn is small, the thickness d is required to be increased. However, if the thickness d is increased, alignment of liquid crystal molecules tends to be difficult, and it tends to be difficult to obtain the desired optical properties. On the other hand, if the Δn value is large, the thickness d is required to be small, but in such a case, it becomes difficult to precisely control the thickness.
Further, in recent years, in order to increase the capacity of optical disks, it has been attempted to shorten the wavelength of a laser beam to be used for writing or reading of information and to further reduce the concavo-convex pit size on optical disks. At present, a laser beam having a wavelength of 780 nm is used for CD, and a laser beam having a wavelength of 660 nm is used for DVD. For optical recording media of next generation, use of a laser beam having a wavelength of from 300 to 450 nm (hereinafter referred to also as a blue laser beam) is being studied. However, conventional materials such as polymer liquid crystals disclosed in JP-A-10-195138 have had a problem that the durability against such a blue laser beam is inadequate.
For example, if an optical element (such as a phase plate) made of an organic substance such as liquid crystal is disposed in an optical system and is used as an optical head device, aberration may sometimes occur as the time passes. This is considered attributable to a damage to the organic substance caused by exposure to the laser beam. Once aberration occurs, when light (a light beam) emitted from a laser light source and passed through e.g. a collimator lens or an optical element, is further passed through an object lens and reaches the surface of the recording medium, the light beam tends to hardly focus to form a beam spot, whereby the efficiency (the light utilization efficiency) for reading or writing of information is likely to be low.
Further, a material having a high refractive index anisotropy is usually required in order to reduce the size and improve the efficiency of an element. Generally, a material having a high refractive index anisotropy tends to have a high refractive index. Further, a high refractive index material has a characteristic such that the wavelength dispersion of the refractive index is large, and thus, the absorption of light with respect to light having a short wavelength tends to be high (i.e. the molar absorbance coefficient of the material tends to be large). Therefore, conventional high refractive index materials have had a problem that they tend to absorb light having a short wavelength such as a blue laser beam, and the light resistance is low.
Accordingly, an optical element such as a diffraction element or a phase plate is required to modulate a laser beam having a wavelength of from 300 to 450 nm, and an optical anisotropic material is desired which is excellent in durability without deterioration even when exposed to a laser beam in such a wavelength zone.
Patent Document 1: JP-A-10-195138